The invention relates generally to nonlinear optical devices, and more particularly to nonlinear optical devices utilizing KTP crystals.
U.S. Pat. No. 3,949,323 to Bierlien and Gier describes the use of potassium titanyl phosphate, KtIOPO.sub.4 (KTP) in nonlinear optical devices. U.S. Pat. Nos. 4,231,838 and 4,305,778 to Gier describe methods of making KTP.
One particular application for KTP as a nonlinear crystal is for optical second harmonic generation, as described in "KTP as a Harmonic Generator for Nd:YAG Lasers", R. F. Belt etal, Laser Focus, October 1985, pp. 110-124. To generate second harmonic, the condition of phase matching must be achieved. Generally, phase matching is of two types: type I wherein the two incident waves have the same polarization, and type II wherein the two incident waves have orthogonal polarization.
The indices of refraction of KTP are known to be essentially temperature independent over a relatively wide temperature range, e.g. as shown in "K.sub.x Rb.sub.l-x TiOPO.sub.4 : A new nonlinear optical material", F. C. Zumsteg etal, J. Appl. Phys., Vol. 47, No. 11, November 1976, pp. 4980-4985, particularly FIG. 6. As a result, the wavelength for phase matching is less sensitive to temperature change than other nonlinear optical materials, i.e. the thermal phase matching bandwidth is relatively large.
Conventionally, KTP crystals are used at room (ambient) temperature. The KTP crystal is cut at the proper angles and aligned for phase matching. Although the indices of refraction and the electro-optic coefficients are relatively constant as a function of temperature, there is no reason in the prior art to operate at higher than room temperature since no increase in conversion efficiency would result (in fact some temperature detuning would occur) and more apparatus would be required. For some crystals such as MfO:LiNbO.sub.3 or BaNaNbO.sub.3 in which phase matching at 1064 nm cannot be achieved at room temperature, it is necessary to operate at elevated temperatures just to produce phase matching. But this does not apply to KTP which is easily angle tuned to phase match at room temperature.
However, when KTP crystals are used for frequency doubling at high power, drift and damage problems occur. For example, in a 2 W frequency doubled modelocked system, or even a 1 W compressed pulse system, these effects may occur. Drift is the falloff of frequency doubled power with time. Beam distortion and damage to the crystal can also occur. It is desirable to reduce or eliminate the problems of drift and damage in a high power laser system using nonlinear KTP crystals for frequency doubling.